Prior U.S. Pat. No. 4,171,206 describes a pressure swing adsorption process (PSA) which can be used for fractionation of a multicomponent gas mixture composed of (1) a bulk primary component, (2) a bulk secondary key component that is more strongly sorbed by the adsorbent than is the primary component and (3) a minor quantity of one or more dilute components less strongly sorbed than the secondary key component. The patent describes by way of example application of the disclosed process to production and recovery of pure hydrogen and CO.sub.2 from a steammethane reformer (SMR) off gas after high temperature shift. Such shifted product typically contains about 20% CO.sub.2, 3.5% CH.sub.4, 0.5% CO and about 76% H.sub.2, available at a pressure of about 250 psig. In a typical run made by the process of the patent such mixed gas was resolved to obtain a primary hydrogen product of 99.999+% H.sub.2 purity at about 87.1% H.sub.2 recovery, and a secondary CO.sub.2 product containing 99.4% CO.sub.2 at about 94.0% recovery. The waste stream contained about 8.1% CO.sub.2, 20.7% CH.sub.4, 5.7% CO and 65.3% H.sub.2. Other runs carried out by the patented method produced on the average a pure primary hydrogen product (99.9+% H.sub.2) at about 91% yield and a secondary pure CO.sub.2 product (99.9+% CO.sub.2) at almost 100% recovery, with a resulting tertiary product comprised of 59% H.sub.2, 36% CH.sub.4, about 4% CO and only a trace of CO.sub.2.
The present invention provides an improvement over the process disclosed in the cited prior U.S. Pat. No. 4,171,206 whereby the yield of recovered high purity primary product, such as hydrogen, is substantially increased.